We reviewed the diagnostic features and clinical course of 140 patients with idiopathic membranous nephropathy who had their index renal biopsies performed at the Mayo Clinic between 1972 and 1984. There were 93 males and 47 females (average age, 50.8 +/- 17 years); 116 patients (83%) had the nephrotic syndrome and 42 (30%) were hypertensive at diagnosis. Eighty-nine patients were not treated with corticosteroid or immunosuppressive drugs and 51 patients were treated mainly with short-term courses of prednisone alone; a minority of patients also received meclofenamate, cyclophosphamide, azathioprine, or chlorambucil. Five-year survival, including patients who received dialysis or a renal transplant, was 85%, 75% at 10 years, and no different from expected survival; there was no difference between untreated and treated groups. Also, there were no differences in the outcomes of renal function and protein excretion between untreated and treated patients. Among 28 patients (20%) who developed end-stage renal disease, 17 showed rapid progression within 2.5 years after diagnosis. Fifteen of the 17 patients were males; all were severely nephrotic and had impaired renal function at diagnosis. Only 1 of 24 patients with nonnephrotic proteinuria at index renal biopsy progressed to end-stage renal disease. Overall, a level of baseline proteinuria of 10 g or more per 24 hours and variable blood pressure control in hypertensive patients were associated with renal progression.